Aluminosilicates are a well known class of molecular sieve materials which have found widespread use as catalysts and absorbents. The basic structure of these crystalline materials comprises SiO4 tetrahedra (which have four oxygen atoms at the apexes with the silicon atom being at the center) and AlO4 tetrahedra (which have four oxygen atoms at the apexes with the aluminum atom being at the center). These tetrahedra are regularly and three dimensionally connected to each other throughout the structure through the sharing of apex oxygen atoms. This arrangement provides a three-dimensional network structure defining pores that differ in size and shape depending on the arrangement of tetrahedra and composition of the structure. In its simplest terms the material may be considered to be a silicate material in which some of the Si4+ ions in the silicate are replaced by Al3+ ions. For each Si4+ ion replaced by an Al3+, the charge must be balanced by having other positive ions such as Na+, K+ or Ca2+ present. It is the presence of framework aluminum in aluminosilicates which is important in providing, for example, the catalytic properties of these materials.
A wide variety of synthetic aluminosilicates can be manufactured through various synthesis routes. It has been relatively easy to manufacture certain aluminosilicates such as ZSM-5, MCM-22, zeolite Beta and ZSM-22 with high SiO2/Al2O3 ratios, that is, aluminosilicates which have relatively low levels of aluminum present in the framework structure. However, it is difficult to achieve low SiO2/Al2O3 ratios of 30 or less, that is, aluminosilicates which have relatively high levels of aluminum. Various attempts to produce such materials have resulted in materials that are non-crystalline and/or are heavily contaminated with other materials.
Large crystal ZSM-5 comprising crystals with a dimension greater than 0.5 μm usually can be prepared without difficulty when the aluminum content in the synthesis mixture is low. As a result, with large crystal ZSM-5, although SiO2/Al2O3 ratios of 100 or higher are relatively easily attainable, SiO2/Al2O3 ratios of 30 or lower are difficult to attain.
EP-A-021674 (Mobil) teaches that large crystal ZSM-5, having a crystal size in excess of 1 μm, can be prepared from a reaction mixture containing tetra alkylammonium cations provided the OH−/SiO2 molar ratio is maintained within the range 0.01-0.07.
U.S. Pat. No. 6,013,239 (Mobil) describes a process for the manufacture of large crystal ZSM-5. The process requires the use of a specific group of organic directing agents namely amino-acids. The resulting ZSM-5 crystalline materials have crystal sizes of 1 to 10 μm but the intermediate products obtained prior to calcination contain organic template in the zeolite pores.
WO 00/37398 (Mobil) describes a process for the manufacture of small crystal ZSM-5 which has a SiO2/Al2O3 molar ratio of less than 25. The preferred ZSM-5 has a molar ratio of 15-20, and specific materials are disclosed having SiO2/Al2O3 molar ratios of 15:1 and 19:1. The synthesis method utilises an amorphous silica-alumina having a SiO2/Al2O3 molar ratio of 10:1 to 25:1.
EP-A-0106552 (Togo) describes a process for the manufacture of zeolites similar to ZSM-5 and ferrierite with high SiO2/Al2O3 molar ratios. The process described is organic template free and requires the crystallization of a homogeneous phase compound comprising granular amorphous aluminosilicates. The resultant aluminosilicates have SiO2/Al2O3 molar ratios in excess of 19.